Ohms. If you wire 4 16 ohm speaker in series, you would have a total of 16+16+16+16= 64 ohms.. When you wire in parallel (just 2 speakers for instance), ohms are dropped. 16x16/16+16.. 256/32=8. Do that again, you have another 8 ohms... 2 sets of 2 speakers in parallel. Run the 2 sets in series...